1. Field of the Invention
The present invention relates to a solid-state image sensing device which includes a photo-electric converting device, a transfer switching device, a floating diffusion area etc. in a pixel.
2. Brief Description of the Related Art
Up to now, a pixel of a metal-oxide-semiconductor (referred to as xe2x80x9cMOSxe2x80x9d)-type solid-state image sensing device has in general a constitution shown in FIG. 15 (PRIOR ART) while each of the other pixels of the same type has a similar constitution to that shown in FIG. 15, respectively. In the figure, a numerical sign 1 stands for a photodiode which is employed as a photo-electric converting device, 2 stands for a transfer switch for transferring electric charges photo-electrically generated by the photo-electric converting device 2, 3 stands for a floating diffusion area whereto the converted electric charge is transferred, 4 stands for an MOS transistor for amplifying a voltage of the floating diffusion area, 5 stands for an output signal terminal for delivering an electric charge signal outside the pixel, 6 stands for a power supply terminal for supplying an electric power to drive the pixel, 7 stands for a constant current source for operating the MOS transistor 4 in source follower configuration and 8 stands for a reset switch for resetting a potential of the floating diffusion area 3 to a voltage of the power supply.
Subsequently, an operational sequence of the MOS-type solid-state image sensor shown in FIG. 15 is described. The floating diffusion area 3 is reset first and is set at a floating state. Then the electric charges stored in the photodiode 1 is transferred to the floating diffusion area 3 by means of turning on the transfer switch 2. The transferred signal is provided as a voltage signal of the floating diffusion area 3 through a source follower circuit which is constituted of, the MOS transistor 4, the power supply 6 and the constant current source 7 from the output terminal 5 toward an outside of the pixel.
Herein, a potential of each portion with respect to an electron is illustrated in FIGS. 16A (PRIOR ART) and 16B (PRIOR ART). FIG. 16A is a view showing a saturation state of the electrons which are stored in the photodiode 1. A quantity of the electrons which fill a potential well is a saturation electric charge quantity while a bottom of the well represents a potential corresponding to a depletion voltage. Further, a bottom of a potential well of the floating diffusion area 3 stands for a potential corresponding to the reset voltage.
On the other hand, FIG. 16B is a view showing another state wherein the electrons are transferred to the floating diffusion area 3 by means of turning on the transfer switch 2. Herein, the term of xe2x80x9csaturation voltagexe2x80x9d is defined by a voltage of the floating diffusion area 3 when the saturation electric charge quantity of the photodiode 1 is transferred to the floating diffusion area 3. Accordingly, if the saturation voltage is higher than the depletion voltage, all of the electric charges stored in photodiode 1 are transferred to the floating diffusion area 3 as shown in the present figure so that the photodiode 1 is depleted, which enables the depletion transfer.
Namely, if the saturation voltage of the floating diffusion area 3 is higher than the depletion voltage of the photodiode 1 as mentioned above, all information included in the photodiode 1 is delivered outward.
However, the conventional technology mentioned above has defects described below: First, it is impossible to attain a constant saturation voltage because of a variance in saturated electric charge quantity induced by dispersed manufacturing parameters which take place in manufacturing steps of the photodiodes 1.
Accordingly, it appears that the depletion voltage turns higher than the saturation voltage in some cases as can be seen from FIG. 17 (PRIOR ART). On that occasion, when the electrons are stored up to a vicinity of the saturation electric charge quantity of the photodiode 1, the stored electrons cannot be thoroughly transferred to the floating diffusion area 3 even if the transfer switch 2 is turned on. Consequently, the electrons are left in the photodiode 1 after turning off the transfer switch 2, which means that all information included in the photodiode 1 is not always exhausted.
Further, the electrons residual in photodiode 1 after turning off the transfer switch 2 are added to information during reading out a subsequent voltage signal by means of turning on the transfer switch 2 so that another defects that not only a constant saturation voltage is unobtainable but also erroneous information is provided to the floating diffusion area 3 are generated.
Accordingly, the conventional technology includes origins which cause after-image and blooming phenomena during operating of the solid-state image sensing device when information of the photodiode 1 is not adequately provided to the floating diffusion area 3 as mentioned above. As a result, the image sensing operation cannot be performed suitably.
To solve the problems mentioned above, an object of the present invention is to control a saturation electric charge quantity of a photo-electric converting device within a chip so that the saturation voltage turns higher than the depletion voltage, thereby to adequately operate a solid-state image sensing device. Another object of the invention is to simply and automatically control the saturation electric charge quantity of the photo-electric converting device.
To satisfy the purposes mentioned above, a solid-state image sensing device according to the present invention is to constitute a photo-electric converting device and an overflow drain device, of which overflow drain level is controlled corresponding to the saturation electric charge quantity of the photo-electric converting device, within a chip.
Further, another constitution according to the present invention is to provide another solid-state image sensing device, comprising:
a photo-electric converting device;
a transfer switch for transferring electric charges photo-electrically generated by the photo-electric converting device;
a floating diffusion area which is the target of the electric charge to be transferred; and
an amplifying part for amplifying the voltage of the floating diffusion area, wherein;
a gate electrode of the transfer switch is employed as a controlled electrode for a lateral overflow drain structure.
In the constitutions according to the present invention mentioned above, an overflow drain level defined individually corresponding to the saturation electric charge quantity in each chip corrects the saturation electric charge quantity within the chip so that the saturation voltage turns higher than the depletion voltage.